Rather then the little white number she sported in Hawaii, Willis wore a great-looking blue bikini, noting of the day's weather: "It is really cold here. I wasn't expecting it."

That night, Willis's outfit was something more modest. With her long hair flowing down her back and a long white dress, black bra and flat shoes ("more comfortable," she noted), she entertained a crowd of 800 locals and guests who gathered for the grand opening of the SLS Hotel South Beach.

Said one guest: "Jayson was so caring and cuddly with Rumer before she performed. They were standing by an elevator nook and he was helping her get ready, giving her confidence, being super supportive. When she sang, he stood in the front row and took pictures."

As for Rumer's personal take on the evening, she told PEOPLE that her parents, Demi Moore and Bruce Willis, are totally supportive.

"My family is behind me with my music," said Willis, 24. "They know it is just another facet of my life."